This invention relates generally to fuses and, more particularly, to fuses with a fuse state indicator.
Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. Fuse end caps typically form an electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. A fusible link is connected between the fuse end caps, so that when electrical current flowing through the fuse exceeds a predetermined limit, the fusible link melts and opens the circuit through the fuse to prevent electrical component damage.
Various types of fuse state indicators have been developed in an attempt to more efficiently locate opened fuses for replacement. For example, U.S. Pat. No. 6,566,996 to Douglass et al., is directed toward a combustible fuse state indicator which is notable both for its low cost construction and its reliability in comparison to other types of indicators. The combustible fuse state indicator of the '996 patent includes a combustible substance located adjacent a transparent lens extending through a side of a rectangularly shaped fuse module. A secondary fuse link extends adjacent the combustible substance and heat associated with opening of the secondary fuse link ignites the combustible substance to reveal a backing layer of a contrasting color. The fuse state indicator of the '996 patent, however, is designed for use with a rectangular fuse module, and implementing such an indicator in other types of fuses presents a number of issues.
For example, in a cylindrical or cartridge fuse, the fuse indicator assembly must be accommodated in a comparatively smaller space than in a rectangular fuse module. Also, the secondary fuse link for the indicator must be electrically connected interior to the fuse body to conductive end caps or terminal elements coupled to the fuse body. Reliably establishing the electrical connection and properly orienting the secondary fuse link with respect to the combustible substance is difficult. Also, due to the curvature of the fuse body, the backing layer beneath the combustible substance can be difficult to see when the combustible substance is consumed.
Still further, in fuses having end caps crimped over a body of the fuse, conductive clips and twisted wire terminations may be used to electrically connect the secondary fuse link of the indicator to the end caps while the end caps mechanically hold the clips and/or terminations in place. In other types of fuses not having end caps, such as knife blade fuses having end bell assemblies, establishing a secure mechanical and electrical connection between the secondary fuse link of the indicator and the end bell assemblies with known clips and terminations is problematic. Relative movement between the end bell assemblies and the indicator as the end bells are installed can damage or break the electrical connections to the indicator.
In some known fuses having end bells and a fuse state indicator, the indicator is soldered to the end bells and an adhesive backing sheet is employed to locate the indicator in a predetermined position with respect to the body. While soldered connections and adhesive backing materials may have some success in establishing electrical connections to the end bells, they do so at an increased cost.
It would therefore be desirable to provide a lower cost fuse state indicator that may be reliably attached to fuses without end caps, such as cylindrical fuses having end bell assemblies.